1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst, and more specifically, to an exhaust gas purifying catalyst that uses zeolite and is excellent in durability, and to a manufacturing method for the same.
2. Description of the Related Art
Exhaust gas purifying catalysts have been known, in which oxides of alumina, ceria and the like supporting noble metal thereon are coated on honeycomb substrates made of ceramics and metal. These exhaust gas purifying catalysts are described in Japanese Patent Laid-Open Publications S53-135898 (published in 1978), S63-88040 (published in 1988), H4-180840 (published in 1992), H5-285386 (published in 1993) and H6-47279 (published in 1994) and Japanese Patent Publication No. 2066821.
These catalysts are generally manufactured in the following manner as described in Japanese Patent Laid-Open Publication H5-285386. First, catalyst material powder of alumina, cerium or zirconium oxide on which noble metal is supported is milled and mixed with water by a ball mill or the like, and thus aqueous slurry is prepared. Then, the slurry is coated on a honeycomb substrate, and then dried and calcined, and thus catalyst-coated layers are formed on the surface of the honeycomb substrate.
Meanwhile, an HC-trap catalyst (three-way catalyst equipped with a HC-trap function) using zeolite for an HC adsorbent has been developed in recent years for the purpose of purifying hydrocarbons emitted in a large amount in a low-temperature range at the time of starting up an engine as an internal combustion engine (hereinafter, referred to as “cold HC”).
Such a HC-trap catalyst as described above has functions of temporarily adsorbing and holding HC emitted in a large amount in the low-temperature range at the time of starting up an engine when the three-way catalyst is not still activated, and gradually desorbing and purifying the HC as the temperature of exhaust gas comes to be elevated to activate the three-way catalyst.
An HC-trap catalyst as shown in Japanese Patent Laid-Open Publication H2-56247 (published in 1990) has been proposed, which includes first layers mainly containing zeolite as an HC adsorbent, the first layers being formed on a honeycomb substrate, and second layers mainly containing noble metals such as Pt, Pd, Rh and the like, the second layers being provided on the first layers.
Moreover, the exhaust gas purifying catalysts, each using such an HC adsorbent as described above, are disclosed in Japanese Patent Laid-Open Publications H7-96177 (published in 1995), H9-38500 (published in 1997), H9-99217 (published in 1997).
Among these catalysts, from a viewpoint of enhancing a heat resistance of zeolite serving as an HC adsorbent, a HC-trap catalyst has been proposed in Japanese Patent Laid-Open Publication H7-96177, which uses an HC adsorbent obtained by carrying out acid treatment against unreformed zeolite and then treating the zeolite under the presence of moisture at a heat resistance temperature thereof or lower. Moreover, from viewpoints of maintaining a trap function of zeolite and controlling deterioration of purifying layers, there has been proposed in Japanese Patent Laid-Open H9-38500, that HC adsorbent layers are formed on a honeycomb substrate, and then, catalyst activation species made of a suitable amount of a noble metal compound and neodymium oxide is supported thereon. Subsequently, from a viewpoint of obtaining good HC-trap adsorbing/desorbing characteristics even if a high temperature of no less than 750° C. is applied, it has been proposed to use, as an HC adsorbent, β-zeolite having a mol ratio of SiO2 to Al2O3 (Si/2Al ratio) to be 100 or more in Japanese Patent Laid-Open Publication H9-99217.
In the exhaust gas catalyst such as a HC-trap catalyst, which contains zeolite, in general, slurry obtained by mixing zeolite and an inorganic binder prepared, the obtained slurry is coated on a honeycomb substrate, and the concerned coated layers are dried and calcined, and thus catalyst layers containing zeolite are formed.